1. Field of the Invention
This invention relates generally to electronic circuits used in automatic test equipment, and relates more particularly to a timing subsystem that includes several test period generators capable of supplying a variety of timing signals to a device under test.
2. Description of the Prior Art
In automatic test equipment used for testing electronic circuits, test patterns of electronic signals are generated and applied to selected input pins of a device under test. The condition of the output pins of the device under test is then detected and compared to a desired condition to determine the functionality or quality of the circuit. The circuitry within the automatic test equipment which establishes the timing of the test patterns is known as a timing subsystem. The timing subsystem must accurately supply timing signals to the device under test, and must be flexible enough to accommodate the timing requirements of a wide range of devices.
Prior art timing subsystems have commonly utilized constant frequency crystal oscillators to generate timing signals. The flexibility of such timing subsystems is limited since the frequencies of the timing signals are defined in terms of fixed multiples and submultiples of the fundamental oscillator frequency.
Test patterns generally include several timing signals, each having a different frequency. Commonly, major clock signals are generated to establish an overall testing rate, and minor clock signals are generated to establish higher frequency timing signals. Prior art timing subsystems using crystal oscillators have provided minor clock signals by using hardware that subdivided the major clock signals and, thus, limited the flexibility of defining the minor clock signals.
Testing requirements sometimes dictate that the timing subsystem be synchronized to the operation of the device under test so that test patterns generated by the testing system are triggered by a signal from the device under test. Prior art timing subsystems using non-resettable crystal oscillators have been inherently inaccurate in synchronizing to external events. It has been customary in such prior art automatic test equipment to pause up to one clock period after the receipt of an external synchronization signal to accommodate signal jitter. This causes a timing uncertainty on the order of one clock period.
Certain devices under test require external clock signals as timing inputs. Prior art timing subsystems typically could not easily provide an external clock signal, so standard practice has been to supply a separate crystal oscillator. This practice, however, increases the cost of testing and restricts flexibility in the operation of the automatic test equipment.
What is needed is an accurate, flexible, and capable timing subsystem for use in automatic test equipment.